A detailed review on the phytochemical profiles and anti-diabetic mechanisms of Momordicacharantia

Diabetes mellitus is the most well-known endocrine dilemma suffered by hundreds of million people globally, with an annual mortality of more than one million people. This high mortality rate highlights the need for in-depth study of anti-diabetic agents. This review explores the phytochemical contents and anti-diabetic mechanisms of M. charantia (cucurbitaceae). Studies show that M. charantia contains several phytochemicals that have hypoglycemic effects, thus, the plant may be effective in the treatment/management of diabetes mellitus. Also, the biochemical and physiological basis of M. charantia anti-diabetic actions is explained. M. charantia exhibits its anti-diabetic effects via the suppression of MAPKs and NF-κβin pancreatic cells, promoting glucose and fatty acids catabolism, stimulating fatty acids absorption, inducing insulin production, ameliorating insulin resistance, activating AMPK pathway, and inhibiting glucose metabolism enzymes (fructose-1,6-bisphosphate and glucose-6-phosphatase). Reviewed literature was obtained from credible sources such as PubMed, Scopus, and Web of Science.


Introduction
Momordica charantia (M. charantia), also known as bitter melon, karela, bitter gourd, or balsam pear, is a medicinal plant from the Cucurbitaceae family; it is predominantly cultivated in Africa, Asia, and South America [1,2]. The name bitter guard or melon is given to it due to the fruit's bitter flavor, which becomes more pronounced as it ripens. Bitter melon is a medicinal plant with diverse beneficial effects [3], although mainly known for its anti-diabetic effects [4]. The anti-diabetic effects of M. charantia can be attributed to its different bioactive substances such as vicine, charantin, glycosides, karavilosides, polypeptide-p, and plant insulin [5]. These bioactive compounds belong to the broad class of phytochemicals: triterpene, protein, steroids, alkaloids, inorganic, lipid, and phenolic compounds [6,7]. M. charantia's anti-diabetic activities are reported in both type 1 and 2 diabetes mellitus. Moreover, all morphological parts of M. charantia demonstrated hypoglycemic activity in normal animals [8], alloxan-induced diabetic [9,10], streptozotocin-induced diabetic model [11,12], as well as diabetes genetic models [13]. In exploratory animal models, M. charantia has shown encouraging impacts in preventing diabetes mellitus and retarding the advancement of diabetic complications, including neuropathy, gastroparesis, nephropathy, waterfall, and insulin obstruction [8].

The global burden of diabetes mellitus occurrence and mortality
Diabetes mellitus (DM), a mixture of heterogeneous problems, is usually characterized by hyperglycemia and glucose bigotry scenes resulting from the lack of insulin production, insulin resistance, or both [14]. Such complications are discernible to the absence of homeostasis in the frameworks liable for the metabolism of biomolecules [15]. DM is a significant precursor of visual impairment, kidney distress, coronary failures, stroke, and lower appendage removal [15]. It is right now a typical and genuine wellbeing concern internationally [16], and the most well-known endocrine dilemma, with approximately 690 million cases prophesied in 2045 [17]. To mitigate against this foreseen spurt in the number of diabetic patients in the near future, it is expedient to accord attention to natural products such as M. charantia that could be maximized in the therapy of DM.   (1) by increasing the rates of glycolysis (2) degradation of TAG by increasing the expression of lipoprotein lipase enzymes (3) enhancement of insulin sensitivity by stimulating adipogenesis and increasing the storage of TAG; this leads to increased fatty acid absorption and reduction in lipid level. [18]. Consecutive use of aqueous and ethanol extracts of M. charantia (200 mg/kg, orally) in alloxan-and streptozotocin-induced diabetic rats resulted in a critical reduction in plasma glucose levels after 21 days, though; the aqueous extract is found more effective [19]. The mash saponin-free methanolic concentrate of M. charantia has a huge antiglycemic impact on fasting and post-prandial conditions in normal, glucose-treated normal and non insulin-dependent diabetes mellitus rats [8]. M. charantia treatment of alloxan diabetic rats impeded cataract development, observed at 100 days in untreated diabetic rats [20]. Another study documented that, regular administration of a high dose of M. charantia extracts to alloxanized diabetic rats (120 mg/kg) for 2-8weeks delayed cataract progression to 140-180 days compared to 90-100 days in control rats [21]. Oral administration of aqueous extracts of M. charantia (400 mg/day for 15 days) to fructose-rich dietary fed rats considerably forestalled hyperglycemia and hyperinsulinemia in comparison with fructose-rich fed untreated groups [22]. Seared M. charantia fruits devoured as a daily food supplement influence a minor but crucial increase in glucose tolerance in diabetic animals/subjects with no expansion in serum insulin levels [23]. In another clinical investigation, a homogenized suspension of M. charantia given to 100 cases of moderate T2DM human subjects resulted in a significant (P < 0.001) decrease in post-prandial serum glucose (86% cases) and fasting glucose (5% cases) [8]. Welihindaa et al. reported glucose tolerance upregulation in 73% of patients with maturity-onset diabetes administered with M. charantia fruit juice [24].

Amino acids
The fruits of M. charantia have been shown to possess certain amino acids. These amino acids are both essential and non-essential amino acids; they include alanine, aspartic acid, butyric acid, g-amino, glutamic acid, isoleucine, leucine, luteolin, methionine, phenylalanine, pipecolic acid, serine, threonine, and valine [75]. All amino acids have a general Figure 6. M. charantia inhibits gluconeogenesis, fatty acid synthesis, and cholesterol synthesis in the liver via the activation of AMPK. AMPK inhibits gluconeogenesis by suppressing the action of CRCTC2 and FOXO1 (genes that are critical in the activation of gluconeogenesis) either directly or indirectly (by increasing the synthesis of P-GSK-3β). The suppression of CRCTC2 and FOXO1 can promote the synthesis of PG-C1α or decrease the action of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). AMPK also inactivates acetyl-CoA carboxylase 1 (ACC1) and 3-hydroxyl-3-methylglutaryl CoA reductase leading to the inhibition of de novo synthesis of fatty acid and cholesterol synthesis. ACC2 is also phosphorylated by AMPK, resulting in increased fatty acid oxidation. molecular structure contains a chiral center and two functional groupsamino and carboxyl groups.

Vitamins
The presence of specific vitamins, which include vitamin A, vitamin E, vitamin C, vitamin B 12, and folic acids, have been confirmed in small quantities in the dried leave of M. charantia. Constrastively, vitamin B 3 , vitamin B 6 , vitamin D, and vitamin K are found in trace amounts in the plant's methanol and pet-ether leaf extract [76].

Peptides and proteins
Proteins, a class of large biomolecule, have diverse biological roles in living organisms. From various morphological parts of M. charantia, a variety of peptides and proteins have been discovered and extracted. Various proteins isolated from M. charantia are highlighted below.

Ribosome inactivating proteins (RIPS)
Ribosome inactivating proteins (RIPs), a class of proteins, have drawn the attention of numerous specialists by virtue of their conceivably exploitable bioactivities. Ribosome-inactivating proteins are toxic Nglycosidases that depurinate eukaryotic and prokaryotic rRNAs, thereby arresting protein synthesis during translation [77]. RIPs are classified as type I or type II based on the number of subunits they contain. Type I RIPs isolated from M. charantia are single-chained. RIPs isolated and charac-

Polypeptide-P
Polypeptide-P is a hypoglycemic glycoprotein peptide. It is derived from M. charantia's fruit, seeds, and tissues [82]. Two types of polypeptide-P with molecular weights of approximately 11 kD (166 amino acids) and 3.4 kD have been isolated from M. charantia [83]. It is crucial in cell recognition and adhesion reactions and has also been isolated from bitter melon [84].

P-insulin
P-insulin, a phytoconstituent of M. charantia, is supposed to be a polypeptide hypoglycemic substance with a molecular weight of~11 kDa and comprises 166 amino acids [83]. P-insulin is found in bitter melon fruits, seeds, and several tissue cultures [3].

Polysaccharides
Polysaccharides rank among the essential bioactive constituents of Momordica charantia. The polysaccharides contents of M. charantia may be influenced by different conditions [92]. These polysaccharides are composed of different saccharide units, including arabinose, galactose, glucose, mannose, and rhamnose, and are thus classified as heteropolysaccharides [93]. The major polysaccharides isolated from M. charantia are shown in Table 1.
Majorly, M. charantia polysaccharides improve cell death, hyperlipidemia, inflammation and oxidative imbalance during myocardial infarction by hindering the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) flagging pathway [97]. M. charantia polysaccharides additionally could improve overall volatile fatty acids generation, regulate the rumen fermentation pathway and impact the quantity of cellulolytic bacteria populace [98].

Mechanisms of anti-diabetic effect of M. charantia
Several scientists have researched the hypoglycemic and antiglycemic impacts of the various concentrates and compounds of M. charantia in human and animal models [8,83]. M. charantia and its various concentrates and extracts applied their hypoglycemic impacts through various pharmacological, physiological, and biochemical modes [99,100]. The reported modes of M. charantia anti-diabetic exercises include hypoglycemic activity [39,94], incitement of glucose to the peripheral and skeletal muscles [95], restriction of intestinal glucose take-up [96,101], hindrance of adipocyte differentiation [102], concealment of main gluconeogenic enzymes [103], incitement of the main biocatalyst of glycolytic pathway [104], and safeguarding of islet β cells and their capacities [105].
In this review, -we explicitly show that M. charantia exhibits its antidiabetic effects through the suppression of mitogen-activated protein kinases (MAPKs) and NF-κβ in pancreatic cells, promotion of glucose and fatty acids catabolism, stimulation of fatty acids absorption, induction of insulin production, amelioration of insulin resistance, activation of AMP --activated protein kinase (AMPK), and inhibition of glucose metabolism enzymes (fructose-1,6-bisphosphate and glucose-6-phosphatase) (Figure 1).

Promotion of glucose and fatty acids catabolism and fatty acid absorption
One study revealed that the M. charantia seeds improve the serum and liver lipid profiles and serum glucose levels by inducing the expression of the peroxisome proliferator-activated receptor gamma (PPAR-γ) gene in the adipose tissue [105]. 9c,11t,13t-CLN is the phytochemical compound involved in the activation of PPAR-γ in M. charantia (Figure 3) [114].
PPAR-γ is a member of PPARs, a subfamily of ligand-activated transcription factors of the nuclear hormone receptors superfamily [115]. PPARs, generally a critical factor in the regulation of the many genes, are involved in coordinating several cellular and metabolic processes such as metabolism of glucose, lipoprotein and triglyceride, energy homeostasis, de novo lipogenesis, uptake, storage, oxidation, and transport of fatty acid, etc. [116,117,118,119,120]. M. charantia seed ameliorates hyperlipidemia and hyperglycemia by acting as a PPAR-γ ligand activator, which stimulates the expression of genes involved in lipid catabolism and glucose utilization (Figure 4) [121]. The stimulation of PPAR-γ has been proven to reduce plasma triglyceride and free fatty acids levels by promoting their breakdown through the induction of lipoprotein lipase [122]. Furthermore, PPAR-γ stimulates cellular differentiation, enhances lipid storage, and regulates insulin activities in the adipose tissue [123]. Activators of PPAR-γ also enhance insulin sensitivity via adipogenesis stimulation and post-prandial fatty acid/triacylglyceride storage within the adipocytes [124].

Induction of insulin production and amelioration of insulin resistance
Jeewathayaparan et al. [125] exhibited that oral administration of M. charantia could prompt insulin emission from endocrine pancreatic β cells; this result was later corroborated by Ahmed et al. [126], who explored the impact of the day to day oral administration of M. charantia natural product juice on the action of α, β and δ cells in the pancreas of STZ-initiated diabetic rodents. Administration of M. charantia alcohol concentration to alloxan-induced diabetic rats shows a strong hypoglycemic effects and significantly improved the islets of Langerhans [127]. Other studies showed that M. charantia could stimulate the emission of insulin from the endocrine pancreas and elicit glucose absorption in the liver ( Figure 5) [101]. We proposed a mechanism by which the aforementioned effects are achieved -the recruitment of GLUT-4 transporter ( Figure 5).
6.4. Activation of AMP-activated protein kinase alpha M. charantia fruits have likewise indicated the capacity to upgrade cells' glucose take-up, advance insulin discharge, and potentiate insulin's impact. Bitter melon's bioactive content enacts a protein called AMPK (AMP-activated protein kinase α), which is notable for controlling energy given foods digestion and empowering forms of glucose take-up, which are impeded in diabetes patients [128]. The mechanisms of anti-diabetes activities of AMPK are well characterized in the liver and the muscle tissues [129]. In the liver AMPK inhibits gluconeogenesis by suppressing the synthesis of key genes such as CREB-regulated transcription co-activator 2 (CRTC2) and forkhead Box O1 (FOXO) [130]. The actions of AMPK in the liver also leads to inhibition of de novo fatty acid synthesis and cholesterol synthesis as well as activation of fatty acid catabolism ( Figure 6) [131]. M. charantia can also induce activation of AMPK in the muscle tissue, resulting primarily into an increment of fatty acid oxidation in the mitochondria and cytoplasm (Figure 7) [132].
6.5. Inhibition of fructose-1,6-bisphosphatase, and glucose-6-phosphatase Fructose-1,6-bisphosphatase and glucose-6-phosphatase activities are repressed by aqueous and alcoholic concentrates of M. charantia [5]. Fructose-1,6-bisphosphatase catalyzes the hydrolysis of fructose-1, 6-bisphosphate to fructose 6-phosphate ( Figure 8) [133]. This reaction occurs in both gluconeogenesis and the Calvin cycle [134]. Fructose-1, 6-bisphosphatase is a rate-limiting enzyme in gluconeogenesis and a key target for T2DM treatment due to the well-known involvement of abnormal endogenous glucose production in the disease's hyperglycemia [135]. Inhibition of fructose-1,6-bisphosphate has been proposed as a potential treatment for T2DM [136,137]. Gluconeogenesis is a major contributor to surfeit glucose in this disease. Reducing its excess would help alleviate the pathology linked to elevated glucose concentrations in the blood and tissues. Inhibiting fructose 1,6-bisphosphatase only affect gluconeogenesis but not glycolysis [138,139,140,141].
Glucose-6-phosphatase (also known as G-6-Pase), which is primarily found in the liver [142], catalyzes the final stage for both glycogenolysis and gluconeogenesis by changing glucose-6-phosphate to inorganic phosphate and glucose ( Figure 8) [143,144], making it an important regulator of blood glucose homeostasis [145]. The enzyme activity is several times higher in diabetic animals and, most likely, in diabetic humans, implying that it may be involved in the increased hepatic glucose production seen in T2DM [146]. Further, in the diabetic condition, the presence of both G-6-Pase (and glucokinase) in pancreatic -cells might result in higher glucose cycling, which can compromise glucose sensing and insulin secretion. Previous studies have shown an association of attenuated insulin production with higher glucose-6-phosphatase activity as well as glucose cycling in T2DM animal models [147,148]. Therefore, M. charantiaa compound that inhibits the glucose-6-phosphatase enzyme complexcould be maximized in the treatment of T2DM.

Future perspective
Approval of any therapeutic substance and its application in pharmaceutical industry for human use is subjected to the success of the substance in clinical trial studies. While M. charantia and its extracts are widely regarded traditionally as a potent anti-diabetic concoction, up to date, there is scarcity of clinical trial studies on the anti-diabetic effects of the plant [8]; hence, the global acceptance of this purported "potent" antidiabetic plant in the treatment of diabetes mellitus is retarded. Unfortunately, the currently approved antidiabetic therapy has not shown maximum success, therefore more clinical studies on the anti-diabetic effects of extract of M. charantia should be encouraged. In addition, attention needs to be paid to the toxicity of M. charantia extract. Many toxicological studies have demonstrated in years past that extracts of M. charantia could be toxic in several organs of the body at varying doses.
More recently a study on the reproductive toxicity of the plant in zebrafish confirm that it is teratogenic and cardiotoxic at certain dose [149]. Also, Abdillah and colleagues reported in 2020 that the administration of ethanolic extract of M. charantia for 28 consecutive days could have a toxic effect the liver and the kidney [150]. These reported toxic effects on vital organs of the body needs to be further elucidated so that a safe dose can be recommended for use [151].

Conclusion
The forgoing shows that M. charantia is a promising antidiabetic plant and could be of great use in the treatment of diabetes mellitus. Being, a phyto-substance, it is easily accessible and relatively cheap; hence, studies should be focused on the development of the plant into a widely acceptable anti-diabetic therapy, especially with a high level of global mortality accorded to diabetes mellitus amidst various anti-diabetic drugs coupled with the outrageous increase in the number of diabetic patients is foreseen.

Author contribution statement
All authors listed have significantly contributed to the development and the writing of this article.

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement
No data was used for the research described in the article.

Declaration of interests statement
The authors declare no conflict of interest.

Additional information
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